In recent years, it is a common practice for the mass production of ingots (also called “slabs”) of steel, various kinds of alloys or the like to use a so-called “continuous casting method”, which involves continuously pouring an alloy or the like in a molten state into a water-cooled mold and gradually drawing a solidified ingot from the mold.
The practical use of continuous casting was originated by continuous casters for billets and blooms, and subsequently, continuous casting of slabs having a large cross-sectional area has become widespread due to the strong demand for energy saving and improvement in productivity.
In order to obtain a high-quality ingot with less nonmetallic inclusions and less component segregation by continuous casting, it is important to appropriately agitate molten metal in the course of solidification. Agitation of molten metal in a slab having a large cross-sectional area and having a large aspect ratio of its cross-sectional shape (for example, the ratio of the length of a long side wall to the length of a short side wall is 5 or more) is liable to cause center segregation and center section cracks and deteriorate processability unlike a slab having a small cross-sectional area and having a substantially square cross-sectional shape, such as blooms and billets, and hence it is required to appropriately agitate the molten metal.
Known examples of technologies of molten metal agitation in continuous casting that deal with the requirement include a method in which an electromagnetic agitation device is provided in the vicinity of a cooled mold or on the back surface of the cooled mold, and molten metal is agitated by using electromagnetic force. The electromagnetic agitation device is, however, extremely expensive, and alternative inexpensive devices for agitating molten metal in a cooled mold have been sought after.
As solutions using inexpensive devices, the methods as disclosed in PTL 1 to 6 have been proposed for blooms and billets of which cross-sectional shapes are substantially square.
PTL 1 proposes a method in which molten metal is discharged from four discharge holes provided rotationally symmetrically at a lower part of an immersion nozzle to a square mold surface in an oblique direction, preferably at an angle of (45±10)°, thereby generating a horizontal swirling flow in molten metal in a mold. This method improved quality of slabs such as blooms and billets, but the degree of its effect was not always considered sufficient. PTL 2 adds improvements to PTL 1 to propose a method in which molten metal is discharged from four discharge holes in discharge directions inclined at a given angle with respect to respective mold surfaces of a square mold rather than being rotationally symmetric, that is, in discharge directions inclined at about ½ of an angle formed by the normal to each side from the center of an immersion nozzle and a diagonal of the square with respect to the normal, thereby causing a horizontal swirling flow in molten metal in the mold and agitating the molten metal in the mold, and PTL 2 indicates that the quality of slabs is improved. These methods, which assume molds for blooms and billets, achieve certain results by supplying molten metal to both of the long sides and the short sides, but in the case of slabs, the methods have a problem in that it is difficult to supply molten metal to end surfaces of the long sides and sufficient agitation effect of molten metal cannot be obtained.
PTL 3 to 6 propose methods in which an immersion nozzle is rotatable such that molten steel is poured into a mold while being swirled, thereby agitating the molten steel in the mold.
PTL 3 proposes a method involving rotatably supporting an immersion nozzle through bearings, providing a clearance between a lower end of a tundish nozzle and an upper end portion of the immersion nozzle, introducing inactive gas to prevent oxygen in the atmosphere from being taken into molten steel through the clearance, and continuously rotating the immersion nozzle at a predetermined number of revolutions by a drive device provided outside. PTL 3 indicates that a horizontal swirling flow is thus generated to agitate molten steel in a mold, and the quality of slabs is improved.
PTL 4 and PTL 5 relate to improvements of PTL 3. PTL 4 proposes a method in which the same mechanism of holding and rotating the immersion nozzle as in PTL 3 is used, but instead of the drive device, reaction of molten steel discharged from discharge holes of the immersion nozzle that are inclined at an angle in a circumferential direction from the center axis with respect to a radial direction is used to continuously rotate the nozzle. PTL 4 indicates that the method of agitating molten steel by rotating the immersion nozzle at the number of revolutions corresponding to the flow rate of the molten steel enables a horizontal swirling flow to be generated to agitate molten steel in a mold, and the quality of slabs is improved. PTL 5 proposes a method involving providing an immersion nozzle with discharge holes at height positions different between right and left discharge holes such that molten steel is poured into a mold from different heights, rotatably supporting the immersion nozzle, and continuously rotating the immersion nozzle at a predetermined number of revolutions by a drive device, thereby efficiently agitating the molten steel. PTL 5 indicates that a swirling flow is generated in the horizontal direction and in the vertical direction to agitate the molten steel in the mold, and the quality of slabs is improved.
In these cases, there is a problem in that when molten steel flows from a tundish nozzle to an immersion nozzle, the pressure in a clearance between the tundish nozzle and the immersion nozzle is decreased in accordance with Bernoulli's law, and a large amount of inactive gas is blown into the molten steel through the clearance, with the result that a large amount of air bubbles is taken in a slab. These methods have achieved effects in terms of molten steel agitation, but when applied to slabs, the methods still have a problem in that it is difficult to supply molten steel to end surfaces of the long sides and sufficient agitation effect of molten metal cannot be obtained.
PTL 6 proposes a twin-roll continuous casting machine configured such that a flange is provided at a lower part of a nozzle extended portion and is brought into slide contact with a flange provided at an upper part of an immersion nozzle, the flanges are pushed against each other by springs or the like, and a drive device is provided to continuously rotate the immersion nozzle at a predetermined number of revolutions. PTL 6 indicates that hot molten steel from a tundish is thus ejected uniformly to the inside of a mold such that molten steel temperatures in the mold are made uniform to prevent the generation of wall shells, and the quality of slabs is improved. If this method is directly applied to an iron-making slab continuous casting machine, however, wear of the above-mentioned slide contact portion becomes a problem. The use of a solid lubricant to achieve lubricity is conceivable, but it is not always effective.
Further, if the methods as disclosed in PTL 3 to 6 in which the discharge directions are continuously rotated to provide a swirling flow to molten steel in a mold are applied to a slab continuous casting machine, there is a problem in that it is difficult to supply molten steel to both o the long sides and the short sides, in particular, difficult to supply molten steel to end surfaces of the long sides, and sufficient agitation effect of molten steel cannot be obtained.
As a solution, PTL 7 proposes a method in which, in a slab continuous casting machine, a two-hole immersion nozzle is mounted and installed such that discharge directions of molten steel fall within the range between the normal from the center axis of the immersion nozzle to the short side of a mold and a diagonal of the mold, thereby supplying molten steel to end surfaces of the long sides while concentrating the molten steel, and smoothly agitating the molten steel. PTL 7 indicates that a molten steel continuous casting method capable of eliminating excessive supply of discharge flows contacting with long-side wall surfaces to prevent breakouts and manufacturing high-quality ingots is provided to further improve the quality of slabs.
In continuous casting, a method of continuing continuous casting by replacing with a ladle filled with new molten steel while using molten steel stored in a tundish as a buffer is referred to as continuous-continuous casting (meaning that continuous casting is continued), and the number of ladles used for continuous-continuous casting is referred to as continuous-continuous number. It is preferred to increase the continuous-continuous number in terms of energy and economics. However, the immersion nozzle in continuous casting is always immersed in molten metal. Oxide slag called “mold powder” is formed in a water-cooled mold for continuous casting in order to achieve lubricity between solidified shell of steel and the water-cooled mold. There is a problem in that a part of the immersion nozzle in contact with the oxide slag causes much erosion and the continuous-continuous number cannot be increased. This problem is solved by appropriately replacing with anew immersion nozzle during continuous-continuous casting. The replacement of immersion nozzles during continuous-continuous casting is called “immersion nozzle quick replacement”, and, for example, an immersion nozzle quick replacement mechanism as disclosed in PTL 8 has been introduced.
Also in continuous casting having such an immersion nozzle quick replacement mechanism, it is required to appropriately agitate molten metal.